U.S. patent number 6,713,519 [Application Number 10/032,207] was granted by the patent office on 2004-03-30 for carbon nanotube-containing catalysts, methods of making, and reactions catalyzed over nanotube catalysts.
This patent grant is currently assigned to Battelle Memorial Institute. Invention is credited to Ya-Huei Chin, Yufei Gao, Yong Wang.
United States Patent |
6,713,519 |
Wang , et al. |
March 30, 2004 |
Carbon nanotube-containing catalysts, methods of making, and
reactions catalyzed over nanotube catalysts
Abstract
Methods have been developed to form catalysts having active
metals disposed on a carbon nanotube coated porous substrate.
Catalysts and reactions over nanotube-containing catalysts are also
disclosed. Results are presented showing enhanced performance
resulting from use of the inventive catalyst. Mesoporous oxide
layers can be utilized to improve catalyst properties.
Inventors: |
Wang; Yong (Richland, WA),
Chin; Ya-Huei (Richland, WA), Gao; Yufei (Blue Bell,
PA) |
Assignee: |
Battelle Memorial Institute
(Richland, WA)
|
Family
ID: |
21863682 |
Appl.
No.: |
10/032,207 |
Filed: |
December 21, 2001 |
Current U.S.
Class: |
518/715; 518/700;
977/835; 977/843 |
Current CPC
Class: |
B01J
20/324 (20130101); B01J 20/3295 (20130101); B82Y
30/00 (20130101); C23C 26/00 (20130101); C10G
2/33 (20130101); B01J 21/18 (20130101); B01J
37/0244 (20130101); B01J 20/28088 (20130101); C01B
32/162 (20170801); F28F 21/02 (20130101); B01J
20/3289 (20130101); B01J 20/205 (20130101); B01J
23/8896 (20130101); B82Y 40/00 (20130101); B01J
19/0093 (20130101); C23C 8/80 (20130101); C04B
41/009 (20130101); B01J 21/185 (20130101); C04B
41/89 (20130101); C10G 2/332 (20130101); B01J
20/28045 (20130101); B01J 37/0225 (20130101); C04B
41/52 (20130101); C23C 8/02 (20130101); B01J
35/04 (20130101); B01J 20/20 (20130101); B01D
69/141 (20130101); B01J 20/28069 (20130101); B01J
20/3204 (20130101); C04B 41/52 (20130101); C04B
41/4535 (20130101); C04B 41/5035 (20130101); C04B
41/52 (20130101); C04B 41/4529 (20130101); C04B
41/4549 (20130101); C04B 41/455 (20130101); C04B
41/5001 (20130101); C04B 41/009 (20130101); C04B
35/00 (20130101); B01J 37/0238 (20130101); B01J
2219/00873 (20130101); Y02C 20/40 (20200801); B01D
2323/283 (20130101); F28F 2255/20 (20130101); B01J
2219/00783 (20130101); Y10S 977/835 (20130101); Y02C
10/08 (20130101); Y02P 20/10 (20151101); Y02P
20/127 (20151101); B01J 2219/00844 (20130101); B01J
37/033 (20130101); C04B 2111/0081 (20130101); Y10S
977/843 (20130101); Y10T 428/30 (20150115); B01J
35/10 (20130101); B01J 2219/00835 (20130101); Y10S
977/782 (20130101); F28F 2260/02 (20130101); Y10S
977/895 (20130101); Y10S 977/742 (20130101) |
Current International
Class: |
B01J
35/00 (20060101); C01B 31/02 (20060101); B01J
19/00 (20060101); B01J 35/04 (20060101); B01J
37/02 (20060101); B01J 37/00 (20060101); C23C
8/80 (20060101); C04B 41/52 (20060101); C04B
41/89 (20060101); C04B 41/45 (20060101); B01J
20/20 (20060101); B01J 20/30 (20060101); B01J
20/32 (20060101); B01J 21/00 (20060101); B01J
21/18 (20060101); C10G 2/00 (20060101); B01J
23/76 (20060101); B01J 23/889 (20060101); C23C
28/00 (20060101); C01B 31/00 (20060101); C23C
8/02 (20060101); F28F 21/00 (20060101); F28F
21/02 (20060101); C23C 26/00 (20060101); B01J
35/10 (20060101); B01J 37/03 (20060101); C07C
027/00 () |
Field of
Search: |
;518/700,715 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Ago, H., et al. "Dispersion of Metal Nanoparticles for Aligned
Carbon Nanotube Arrays." pp. 79-81. 2000. .
Burghard, M., et al. "Assembling Techniques for Micellar Dispersed
Carbon Single-Walled Nanotubes." pp. 44-49. 1998. .
Gao, Y., et al. "Carbon Nanotubes on a Substrate and Method of
Making." (PNNL Application for E-1743) pp. 1-19. 1999. .
Xie, SS., et al. "Synthesis and Characterization of Aligned Carbon
Nanotube Arrays." pp. 1135-1138. 1999. .
International Search Report from PCT/US 02/40874 (Jun. 2003). .
Duxiao et al., "Catalytic growth of carbon nanotubes from the
internal surface of Fe-loading mesoporous molecular sieves
materials," Materials Chem. And Phys., 69, 246-251 (2001). .
Johnson et al., "Adhered supported carbon nanotubes," J.
Nanoparticle Research, 3, 63-71 (2001). .
Huczko, "Template-based synthesis of nanomaterials," Appl. Phys. A.
70, 365-376 (2000). .
Ago et al., "Dispersion of metal nanoparticles for aligned carbon
nanotube arrays," Appl. Phys. Lett., 77, 79-81 (Jul., 2000). .
Xie et al., "Synthesis and Characterization of Aligned Carbon
Nanotube Arrays," Advanced Materials, 11, 1135-1138 (1999). .
Xu et al., "Controlling growth of field emission property of
aligned carbon nanotubes on porous silicon substrates," Appl. Phys.
Lett., 75, 481-483 (1999). .
Burghard et al., "Assembling techniques for micellar dispersed
carbon single-walled nanotubes," Electronic Properties of Novel
Materials: XII, ed. Kuzmany, Am. Inst. Phys., 44-49 (1998). .
U.S. patent application Ser. No. 10/036,332, filed Dec. 24,
2001..
|
Primary Examiner: Parsa; J.
Attorney, Agent or Firm: Harrington; Todd J. Rosenberg;
Frank S.
Claims
We claim:
1. A method of conducting a catalyzed Fischer-Tropsch reaction,
comprising: passing a gaseous composition comprising Co and
hydrogen into a catalyst comprising: a support material having
through-porosity and an average pore size, as measured by
microscopy, of at least 1 micrometer (.mu.m); a layer comprising
carbon nanotubes on the support material; and a surface-exposed
catalyst composition; reacting the gaseous composition to form at
least one hydrocarbon.
2. The method of claim 1 wherein an oxide is disposed over the
nanotubes.
3. The method of claim 2 wherein the oxide layer comprises a
mesoporous layer.
4. A Fischer-Tropsch process comprising: passing a gaseous
composition over a catalyst comprising: a support material having
through-porosity; a layer comprising carbon nanotubes on the
support material; and a surface-exposed catalyst composition;
wherein the gaseous composition comprises CO and hydrogen; and
forming a hydrocarbon.
5. The process of claim 4 wherein the catalyst composition
comprises Co.
6. The method of claim 4 comprising: passing CO and hydrogen into a
reaction chamber; wherein the catalyst is disposed within the
reaction chamber; and reacting the CO and hydrogen in the reaction
chamber to produce at least one hydrocarbon.
7. The method of claim 6 wherein the reaction chamber has an
interior with a cross-sectional area and the catalyst occupies at
least 80% of said cross-sectional area.
8. The method of claim 7 wherein the reaction chamber is a
microchannel and the catalyst comprises a monolith.
9. The method of claim 6 wherein the Fischer-Tropsch reaction
occurs in an array of microchannel reaction chambers.
10. The method of claim 9 wherein the array of reaction chambers is
in thermal contact with at least one microchannel heat
exchanger.
11. The process of claim 4 wherein the catalyst is characterizable
by a reactivity such that, when tested at 265.degree. C., at 16
atm, a H.sub.2 /CO ratio of 2, and a 250 msec contact time, the
catalyst exhibits: a CO conversion of at least 25%, a methane
selectivity of less than 30%; and a specific activity (defined as
mmol CO converted per gram of total metal (which does not include
metal in oxide support) per hour) of at least 1500.
12. The process of claim 4 having a CO conversion of at least 25%,
a methane selectivity of less than 30%; and a specific activity
(defined as mmol CO converted per gram of total metal (which does
not include metal in oxide support) per hour) of at least 2000.
13. The process of claim 4 having a CO conversion of at least 25%,
a methane selectivity of less than 30%; and a specific activity
(defined as mmol CO converted per gram of total metal (which does
not include metal in oxide support) per hour) of 1800 to about
2400.
14. A method of converting CO and hydrogen, comprising: passing Co
and hydrogen into a reaction chamber; wherein a catalyst is
disposed within the reaction chamber; wherein the catalyst
comprises: a support; nanotubes disposed over said support; an
oxide disposed over the nanotubes; and a catalyst composition
disposed over the oxide; and reacting the at least one reactant in
the a reaction chamber to produce at least one product.
15. The process of claim 4 wherein the support material has an
average pore size, as measured by mercury porisimetry and nitrogen
adsorption, of 0.3 to 200 .mu.m.
16. The process of claim 12 wherein the gaseous composition
contacts the catalyst for 250 ms or less.
17. The method of claim 6 wherein the CO and hydrogen contact the
catalyst for 250 ms or less.
18. The method of claim 17 having a co conversion of at least 25%,
a methane selectivity of less than 30%; and a specific activity
(defined as mmol CO converted per gram of total metal (which does
not include metal in oxide support) per hour) of at least 2000.
19. The process of claim 4 wherein the support comprises a
honeycomb, foam or felt.
20. The method of claim 14 wherein the support comprises a
honeycomb, foam or felt.
21. The method of claim 14 wherein the catalyst composition
comprises Co.
Description
FIELD OF THE INVENTION
The present invention relates to catalysts containing carbon
nanotubes, methods of making catalysts containing carbon nanotubes
on porous substrates, systems employing carbon-nanotube-containing
catalysts, and reactions catalyzed in porous carbon
nanotube-containing catalysts.
INTRODUCTION
Catalysts are crucially important in controlling chemical reactions
in virtually all aspects of our lives. For example, catalysts are
used to lower the temperature, increase the rate, and control the
products in chemical reactions. While catalysts can be liquids or
gases, solid catalysts are especially attractive for commercial
applications because they are easy to store and transport, are
readily separated from product streams, tend to be more
environmentally benign, and can provide superior performance and
greater control of a reaction.
A well-known problem with solid catalysts is slow heat and/or mass
transfer. That is, with solid catalysts and systems employing solid
catalysts, the speed of a chemical reaction can be limited by the
time necessary for heat to travel to or from the catalyst or by the
time needed for chemicals to get to and from the catalyst.
For many years, scientists and engineers have sought better
catalyst materials with improved heat and/or mass transport
properties. While there are probably thousands of publications and
patents that address these problems, two recent patents are
discussed herein.
In one approach, van Wingerden et al., in U.S. Pat. No. 6,099,965,
described methods of making catalysts having desirable heat
transport properties. In one example, particles of an iron-chromium
alloy are placed in a steel pipe and sintered in a hydrogen
atmosphere. The sintered particles are then heat treated in air and
then treated with a suspension of alumina and Fe.sub.2 O.sub.3
/Cr.sub.2 O.sub.3.
Tennet et al., in U.S. Pat. No. 6,099,965, described a catalyst
comprising a rigid carbon nanotube structure and a catalytically
effective amount of a catalyst supported thereon. Numerous
advantages of this structure including enhanced heat and mass
transfer are dicussed (see col. 16, lines 8-65).
Despite the prior work, there remains a need for novel solid
catalyst materials that have superior heat and/or mass transport
capabilities.
SUMMARY OF THE INVENTION
In a first aspect, the invention provides an engineered catalyst
that includes a support material having through-porosity (defined
as discussed below), a layer comprising carbon nanotubes on the
support material; and a surface-exposed catalytically-active
composition.
In another aspect, the invention provides catalyst including a
support; nanotubes dispersed over the support; and a
catalytically-active composition dispersed over the nanotubes.
In yet another aspect, the invention provides a method of forming a
porous carbon nanotube containing catalyst structure. In this
method, a large pore support is provided having through porosity.
Carbon nanotubes are formed over the large pore support, and a
catalyst composition is deposited over the carbon nanotubes.
The invention also includes methods of conducting catalyzing
chemical reactions in which one or more reactants are contacted
with any of the carbon nanotube containing catalysts described
herein. In this method, the one or more reactants react to form a
product. The catalyst catalyzes the reaction relative the same
reaction conducted in the absence of a catalyst. For example, the
invention provides a Fischer-Tropsch process in which a gaseous
composition, comprising CO and hydrogen, is passed over any of the
carbon nanotube containing catalysts described herein.
The invention also provides a catalytic process for aqueous phase
hydrogenations to produce higher value chemical products from
biomass feedstock.
In another aspect, the invention provides a process of making a
porous, carbon nanotube-containing structure, comprising: providing
a support material having through-porosity; depositing seed
particles on the support material to form a seeded support
material; and heating the support material and simultaneously
exposing the seeded support to a carbon nanotube precursor gas to
grow carbon nanotubes on the surface of the seeded support
material.
In another aspect, the invention provides a porous
carbon-nanotube-containing structure that includes a large pore
support having through porosity; and carbon nanotubes disposed over
the large pore support.
In still another aspect, the invention provides a method of making
a carbon-nanotube-containing structure in which a surfactant
template composition (a composition containing a surfactant and
silica or silica precursors) is applied onto a support. Carbon
nanotubes are then grown over the layer made from the surfactant
template composition.
The invention also provides processes of using carbon
nanotube-containing structures. Preferably, any of the carbon
nanotube-containing structures described herein can be used in
processes including: adsorption, ion exchange, separation of
chemical components, filtration, storage of gases (for example,
hydrogen or carbon dioxide), distillation (including reactive
distillation), as a support structure for chemical or biological
sensors, and as a component in a heat exchanger. Features of carbon
nanotube-containing structures that make these structures
particularly advantageous include: high surface area, excellent
thermal conductivity, capillary force for enhanced condensation,
and good attractive force for certain organic species.
Thus, the invention provides a method of adsorbing a chemical
component in which a chemical component is contacted with a carbon
nanotube-containing structure and the chemical component is
adsorbed on the surface of the carbon nanotube-containing
structure. A preferred chemical species is hydrogen. In a preferred
embodiment, the exterior surface of the carbon nanotube-containing
structure is a palladium coating. In preferred embodiments, the
adsorption is run reversibly in a process such as pressure swing or
temperature swing adsorption. This method is not limited to
adsorbing a single component but includes simultaneous adsorption
of numerous components.
Similarly, the invention provides a method of separating a chemical
component from a mixture of components. "Mixture" also includes
solutions, and "separating" means changing the concentration of at
least one component relative to the concentration of at least one
other component in the mixture and preferably changes the
concentration of at least one component by at least 50% (more
preferably at least 95%) relative to at least one other
component--for example reducing the concentration of a 2M feed
stream to 1M or less. Particular types of separations include
filtration, selective adsorption, distillation and ion exchange.
Filtering can be accomplished, for example, by passing a mixture
having at least two phases through a porous carbon
nanotube-containing structure where at least one of the phases gets
physically caught in the structure. A carbon nanotube-containing
structure with surface-exposed carbon nanotubes can function
efficiently for the separation of some organics because the
nanotubes can be hydrophobic while organics can be adsorbed quite
well. For ion exchange it is desirable to coat the surface with an
ion exchange agent.
The preparation of porous materials, such as foams, coated with
carbon nanotubes and a high-surface area metal oxide coating, can
be difficult. The locally aligned nanotubes exhibit high surface
Van der Waal forces and hydrophobic properties. Conventional wash
coating of metal oxides using aqueous based solution often results
in a non-uniform coating or poor adhesion onto the nanotubes. We
have developed treatment methods to modify the surface properties
of the nanotubes, making this new class of materials possible for
application as engineered catalyst structure. We have fabricated
carbon nanotube-based engineered catalyst and have demonstrated its
performance for Fisher-Tropsch reaction in a microchannel reactor.
Under operating conditions typical of microchannel reactors with
minimal heat and mass transfer limitations, it was found that the
integrated nanotubes substrate has further improved the
performance, as indicated by enhanced reaction rate and improved
product selectivity. This concept can also be applied toward
conventional reactors, which operate under severe heat and mass
transfer inhibitions with catalyst performance far less than that
predicted from the intrinsic kinetics.
Various embodiments of the present invention can offer numerous
advantages, including: creating larger pores through which
reactants/products transport to the catalytic sites, improved heat
transport, controlling the direction of heat transport, enhanced
surface area, excellent thermal stability, excellent thermal
conductivity, reduced mass transfer limitations, utility in
microreactors, ready adaptability in fixed-bed type reactors, and
increased catalyst loading levels.
The surface area enhancement that arises from these nanoscale
fibers can greatly increase the catalyst site density within a
fixed reactor volume. The potential to create larger pore size
naturally generated from the interstices between carbon nanotubes
can be beneficial for reactions involving both gas and liquid
phases liquid reactants or products on a solid catalyst, since the
transport of gas phase molecules through the liquid phase inside
the pores is often the rate-limiting step which not only hinders
the reaction rate but also adversely affects product
selectivity.
In this application, "pore size" and "pore size distribution" can
have different meanings as explained below. "Pore size" can be
measured by (optical or electron) microscopy where pore size
distribution and pore volume are determined statistically from
counting in a field of view (of a representative portion of the
material) and pore size of each pore is the average pore diameter.
Pore size is determined by plotting pore volume (for large pore
materials the volume of pores having a size of less than 100 nm can
ignored) vs. pore size and "average pore size" is the pore size at
50% of the existing pore volume (e.g., for a material that has a
40% pore volume, the "average pore size" is the size of the largest
sized pore that adds with all smaller sized pores to reach 20% pore
volume). Where practicable, the pore size and pore volume are
measured on a cross-section of the material that may be obtained
with a diamond bladed saw. For an isotropic material any
representative cross-section should produce the same results. For
anisotropic materials the cross-section is cut perpendicular to
maximum pore length.
Alternatively, pore size and pore size distribution can be measured
by nitrogen adsorption and mercury porisimetry.
A "large pore" support (or other material) is a support that is
characterized by the presence of pores having a pore size
(diameter) of at least 100 nm, more preferably at least 1 .mu.m,
and in some embodiments 500 nm to 400 .mu.m. Preferably, these
supports have through porosity, such as in honeycombs, foams or
felts.
"Through porosity" means that (1) when a "through porosity"
material is sized (sized means cut or grown--that is, a through
porosity material need not be 1 cm in length, but for testing
purposes could be grown or manufactured) to a length of 1 cm (or at
least 0.1 cm if 1 cm is unavailable) and oriented in the direction
of maximum flow, a measurable amount of argon gas will flow through
the intact material, and (2) a cross-section taken at any point
perpendicular to flow (for example, where the material is disposed
within a reactor) shows the presence of pores, and, in the large
pore materials, the presence of large pores. In the present
invention, the interstices between packed, unsintered powder
particles or pellets do not qualify as through porosity (although
powders sintered to form larger materials would qualify). By
definition, materials having only pitted surfaces (such as anodized
aluminum) do not have through porosity, and mesoporous silica (by
itself) does not have through porosity. Anodized aluminum is not a
through porosity material.
A "carbon nanotube" is primarily or completely carbon in a
substantially cylindrical or rod-like form having a diameter of
less than 200 nm, preferably in the range of 4 to 100 nm.
"Nanotubes" may include both tubes and rods.
An "engineered catalyst" means a catalyst having a porous support,
carbon nanotubes, and a catalytically active material disposed over
at least a portion of the nanotubes.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1a is a scanning electron micrograph (SEM) of a metal alloy
foam.
FIG. 1b shows an SEM of a metal foam having a coating of carbon
nanotubes.
FIG. 1c is a higher magnification view of the foam of FIG. 1b.
FIG. 1d is a higher magnification view of the foam of FIG. 1b.
FIG. 2a shows an SEM of a foam having a coating of carbon
nanotubes.
FIG. 2b shows an SEM of a foam having a coating of carbon nanotubes
after a longer exposure to conditions for nanotube growth.
FIG. 2c shows an SEM of a foam having a coating of carbon nanotubes
after a still longer exposure to conditions for nanotube
growth.
FIG. 3a shows an SEM of a ceramic foam having a coating of carbon
nanotubes.
FIG. 3b is a higher magnification view of the foam of FIG. 3a.
FIG. 4a shows an SEM of a metal foam having a coating of carbon
nanotubes and a surface wash coat of alumina.
FIG. 4b is a higher magnification view of the foam of FIG. 4a.
FIG. 4c is a higher magnification view of the foam of FIG. 4a.
FIG. 5 is an illustration of a microreactor 10, including reaction
chamber 18 in which reactants are converted to products 24. An
optional thermally conductive separation plate could be used to
separate the reaction chamber from microchannel heat exchanger 12,
through which flows heat exchange fluid 26.
DETAILED DESCRIPTION OF THE INVENTION
In some embodiments, support materials of the present invention
preferably have through-porosity, preferably these pathways are
sufficiently large to allow molecular diffusion at room
temperature. In some preferred embodiments, the support is a porous
material having a pore volume of 5 to 98%, more preferably 30 to
95% of the total porous material's volume. Preferably, at least 20%
(more preferably at least 50%) of the material's pore volume is
composed of pores in the size (diameter) range of 0.1 to 300
microns, more preferably 0.3 to 200 microns, and still more
preferably 1 to 100 microns. Pore volume and pore size distribution
are measured by Mercury porisimetry (assuming cylindrical geometry
of the pores) and nitrogen adsorption. As is known, mercury
porisimetry and nitrogen adsorption are complementary techniques
with mercury porisimetry being more accurate for measuring large
pore sizes (larger than 30 nm) and nitrogen adsorption more
accurate for small pores (less than 50 nm). Pore sizes in the range
of about 0.1 to 300 microns enable molecules to diffuse molecularly
through the materials under most gas phase catalysis conditions.
Preferred forms of the support are foams, felts (i.e., nonwoven
accumulations of strands or fibers), meshes, membranes, and
honeycombs. In some particularly preferred embodiments, the support
has tortuous porosity with interconnected passages, such as in
foams; this tortuosity creates more desirable flow patterns for
better mixing and heat transfer. Another preferred form is a
microchannel (that is, a channel having a width and/or height of 1
mm or less) array. Other embodiments of the invention can have
supports of larger dimensions, for example a minimum dimension of 1
cm (i.e., each of height, length and width are more than 1 cm).
The support can be made of a variety of materials such as ceramic,
but in embodiments requiring rapid heat transport, the support
preferably is a thermally conductive material such as a metal. In
some particularly preferred embodiments, the support is stainless
steel, or an alloy such as monel. In other embodiments, preferred
support materials include cordierite, silica, alumina, rutile,
mullite, zirconia, silicon carbide, aluminosilicate, stabilized
zironia, steel and alumina-zirconia blend. For use in hydrothermal
conditions, preferred supports include zirconia and carbon.
In other embodiments, where through porosity is not necessary, the
support can be a thin membrane of anodized aluminum (a macroporous
aluminum oxide membrane) or other macroporous membrane (with a
volume average pore diameter of at least 20 nm, for example,
commercially available ceramic membranes with the macropores
straight through the membrane thickness) that is optionally treated
with a surfactant template composition such that mesoporous silica
substantially fills the macropores. Carbon nanotubes are applied to
form a membrane that can be used, for example, as a molecular
sieve, an adsorbent, or treated with an ion exchange medium. The
inventive structures made with an adherent mesoporous silica layer
disposed between the support and the carbon nanotubes may or may
not have through porosity. "Substantially fills" the macropores
means that the mesoporous silica extends completely across the
diameter of a macropore but does not necessarily completely fill
each macropore.
The carbon nanotubes are preferably at least 90 mol % C, more
preferably at least 99 mol % C. The nanotubes may have a metallic
nanoparticle (typically Fe) at the tips of the nanotubes. The
nanotubes have a length to width aspect ratio of at least 3; more
preferably at least 10. The nanotubes preferably have a length of
at least 1 .mu.m, more preferably 5 to 200 .mu.m; and preferably
have a width of 3 to 100 nm. In some preferred embodiments, as
measured by SEM, at least 50% of the nanotubes have a length of 10
to 100 .mu.m. Preferably, of the total carbon, as measured by SEM
or TEM, at least 50%, more preferably, at least 80%, and still more
preferably, at least 90% of the carbon is in nanotube form as
compared to amorphous or simple graphite form.
Depending on the intended use, the distribution of nanotubes can be
tailored to obtain the desired characteristics, for example,
surface area and thermal transport. The nanotubes preferably have
an average separation (from central axis to central axis, as
measured by SEM) of from 10 to 200 nm, more preferably 20 to 100
nm. Having close neighbors, means that the nanotubes will be highly
aligned. In some preferred embodiments, the nanotubes are
sufficiently dense to cover the underlying support, as measured by
SEM. In some preferred embodiments, the material includes nanotubes
arranged in clumps on the support where there is a high degree of
nanotube alignment within each clump (see, e.g., FIG. 1b).
Preferably, the surface area of the article, as measured by
BET/N.sub.2 adsorption, is at least 50 m.sup.2 /g nanotubes, in
some embodiments 100 to 200 m.sup.2 /g nanotubes; and/or at least
10 m.sup.2 /g (nanotubes+support), in some embodiments 10 to 50
m.sup.2 /g (nanotubes+support). Size and spacing of the carbon
nanotubes can be controlled by control of the surfactant template
composition; for example, larger diameter nanotubes can be obtained
by use of larger surfactant molecules.
A "catalyst composition" is a composition of matter that will
catalyze a chemical reaction. Preferred embodiments of the
invention include a catalyst composition that is exposed on at
least one surface. The invention is not limited to specific
catalyst types. In applications where a catalyst composition (or a
catalyst composition precursor) is deposited directly on the
nanotubes, the catalyst composition may be any of the catalysts
used on carbon supports. Additional layers can be deposited on the
nanotubes to support a desired catalyst. Typical catalysts include
metals and metal oxides. Especially preferred catalysts include:
Fischer-Tropsch catalysts (to cite one example, Co-based
catalysts), and steam reforming catalysts. Knowledge of the
scientific literature and routine experimentation can be used by
skilled workers to select appropriate catalyst compositions for
reactions such as acetylation, addition reactions, alkylation,
dealkylation, hydrodealkylation, reductive alkylation, amination,
aromatization, arylation, carbonylation, decarbonylation, reductive
carbonylation, carboxylation, reductive carboxylation, reductive
coupling, condensation, cracking, hydrocracking, cyclization,
cyclooligomerization, dehalogenation, dimerization, epoxidation,
esterification, exchange, halogenation, hydrohalogenation,
homologation, hydration, dehydration, hydrogenation,
dehydrogenation, hydrocarboxylation, hydroformylation,
hydrogenolysis, hydrometallation, hydrosilation, hydrolysis,
hydrotreating, hydrodesulferization/hydrodenitrogenation (HDS/HDN),
isomerization, methanol synthesis, methylation, demethylation,
metathesis, nitration, partial oxidation, polymerization,
reduction, steam and carbon dioxide reforming, sulfonation,
telomerization, transesterification, trimerization, water gas shift
(WGS), and reverse water gas shift (RWGS).
The inventive structures may include additional materials such as
carbide, nitride, sulfide, or oxide layers or metal layers. A
particularly preferred interlayer material is a mesoporous thin
silica film, preferably disposed between the support and the
nanotubes. Mesoporous materials are described in publications such
as U.S. Pat. No. 5,645,891, which is incorporated herein, and D.
Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, and G. D. Stucky,
Adv. Mater., 1998, Vol. 10, No. 16, P1380-1385. These mesoporous
interlayers offer numerous advantages including enhanced surface
area (typically 800 to 1000 m.sup.2 /g silica) and providing
uniform microenvironments for catalysis (for example, while the
supports can be anisotropic, the mesoporous interlayer can put the
same material and same pore size throughout the structure--this
promotes uniform deposition of nanoparticle seeds and, therefore,
uniform nanotubes). In some preferred embodiments, the mesoporous
film is codeposited with metallic nanoparticles, such as Fe
nanoparticles, that serve as seeds for nanotube growth.
In another preferred embodiment, an oxide layer or layers are
disposed between the support and the nanotubes. A thin oxide layer
can be formed on metal supports, for example, by heat treatment in
the presence of oxygen. This oxide layer can improve adhesion of
subsequent oxide layers, and protect the underlying support from
degradation. Alternatively, or in addition, a metal oxide layer can
be deposited over the support. For example, an alumina layer can be
deposited (preferably a dense layer applied by chemical vapor
deposition) onto the support before applying the nanotubes. The
oxide layer(s) may enhance adhesion between the ceramic-metal
interface, as well as protecting the underlying support from
degradation during preparation or use. An oxide layer is especially
desirable over a metal support. The thickness of the oxide layer(s)
in some embodiments is preferably less than about 200 nm, and, in
some embodiments is in the range of 0.05 to 5 .mu.m, and in some
embodiments is in the range of 100 to 1000 nm. Ideally, these oxide
layer(s) should be thick enough to promote formation of a dense
nanotube layer, but thin enough to have little adverse affect on
thermal conductivity.
An oxide layer can be disposed over the carbon nanotubes by
washcoating or vapor coating. In a preferred embodiment, the
nanotubes surface can be oxidized (or partly oxidized). Oxidation
can be done, for example, by exposure to air at elevated
temperature. Preferred conditions are 350 to 500.degree. C. for at
least one minute; more preferably 400 to 500.degree. C. for 2 to 50
minutes. Other oxidation methods could alternatively be used, for
example, treatment with an acidic solution, or coating with a layer
of vapor deposited hydrophilic material. It is believed that these
treatments modify the nanotube surfaces such that the washcoating
solution is absorbed into the interstices during subsequent coating
steps.
In especially preferred embodiments, an oxide layer is, or
includes, a mesoporous silica layer. A mesoporous silica layer may
be formed along with nanoparticles which seed the growth of carbon
nanotubes. Each mesoporous silica layer preferably has a thickness
of between 0.5 and 3 .mu.m.
In another embodiment a carbon nanotube-containing includes a layer
of an electroactive compound, preferably an electroactive polymer,
disposed either between the support and the nanotubes or over the
nanotubes. Such structures can be used to separate chemical
components. In these embodiments, the support is preferably
electrically conductive such as a metal mesh.
Some inventive catalysts or carbon nanotube-containing structures
can be characterized by their properties. In this context,
catalysts are defined to include all the components including
support, nanotubes and catalytically active component (such as a
reduced metal). The catalysts or carbon nanotube-containing
structures preferably have through porosity, preferably have large
pores, and are preferably a porous material having a pore volume of
5 to 98%, more preferably 30 to 95% of the total engineered
catalyst's or carbon nanotube-containing structure's volume.
Preferably, at least 20% (more preferably at least 50%) of the
material's pore volume is composed of pores in the size (diameter)
range of 0.1 to 300 microns, more preferably 0.3 to 200 microns,
and still more preferably 1 to 100 microns. The engineered catalyst
or carbon nanotube-containing structures preferably has a surface
area of at least 0.05 m.sup.2 /g, more preferably at least 0.5
m.sup.2 /g, and in some embodiments between 0.1 and 100 m.sup.2 /g.
The catalyst or carbon nanotube-containing structure is preferably
not a powder, and more preferably is a monolith having a volume
(including voids) of at least 5 mm.sup.3, and in some embodiments 5
to 5000 mm.sup.3. In preferred embodiments, a reactor could have
monolith packing (that would have many small monoliths) or large
catalyst inserts, where a single piece is loaded into the reactor.
Alternatively, or carbon nanotube-containing catalysts can be in
the form of pellets or powders and used in conventional fixed or
fluidized bed reactors.
The inventive catalysts can also be characterized by their
reactivity. For example, a Fischer-Tropsch catalyst according to
the present invention, when tested at 265.degree. C., at 16 atm, a
H.sub.2 /CO ratio of 2, and a 250 msec contact time, preferably
exhibits: a CO conversion of at least 25%, more preferably at least
35%, and in some embodiments about 30 to about 45%, a methane
selectivity of less than 35%, more preferably less than 30%, and in
some embodiments about 35 to about 25%; and a specific activity
(defined as mmol CO converted per gram of total metal (which may
include Co+Re, etc. but does not include metal in oxide support)
per hour) of at least 1500, more preferably at least 2000, and in
some embodiments 1800 to about 2400.
In some preferred embodiments, the fabricated catalyst or carbon
nanotube-containing structures contains 0.1 to 20 weight %
carbon.
The inventive structures are preferably disposed within
microdevices such as microreactors with integral or adjacent heat
exchangers, preferably microchannel heat exchangers. Examples of
reactor configurations are disclosed in U.S. patent application
Ser. No. 09/640,903, which is incorporated herein by reference as
if reproduced in full below. In one preferred embodiment, the
invention comprises a reaction chamber and at least one adjacent
heat exchange chamber. The catalyst (including support, nanotube
layer and catalyst) can be sized to match the flow path of the
reaction chamber such that flow is substantially through the pores
of, rather than around the body of, the catalyst. In some preferred
embodiments, the engineered catalyst (including voids within the
catalyst) occupies at least 80%, more preferably at least 95%, of a
cross-sectional area of the interior of a reactor chamber.
Preferably, the engineered catalyst is a single piece (monolith) or
line of pieces in the reaction chamber occupying at least 80 or 95%
of the cross-sectional area of the interior of a reactor chamber.
Preferably, the engineered catalyst is a removable piece or pieces
rather than a coating.
Other devices for alternative embodiments of the invention include
devices for distillation (such as described in U.S. patent
application Ser. No. 10/011,386 by TeGrotenhuis et al. filed Dec.
5, 2001 which is incorporated herein by reference as if reproduced
in full below) including reactive distillation, gas storage (such
as devices for swing adsorption described in U.S. Pat. No.
6,508,862 filed Apr. 30, 2001 which is incorporated herein by
reference as if reproduced in full below)
Support materials can be obtained commercially. Supports can also
be made by known techniques. Optionally, an intermediate layer or
layers can be applied to the support. The intermediate layer(s) can
be applied by known methods such as wash coating and vapor
deposition. The support, or intermediate layer if present, is then
seeded with nanoparticles, preferably iron nanoparticles. This can
be achieved by applying an aqueous metal solution followed by
calcination to form the nanoparticles.
In some particularly preferred embodiments, a mesoporous silica
layer is deposited on the support. See the Examples for a
description of a suitable technique for depositing a mesoporous
layer. The mesoporous silica layer can be formed from compositions
containing silica precursors and surfactant. To make nanoparticles
for seeding nanotubes, the composition may also contain a
nanoparticle precursor such as a transition metal complex. In
preferred embodiments, a composition comprising surfactant and
silica precursor, or a composition containing the transition metal
complex, or both, are aged before they are combined (aging can be,
for example, at least 5 minutes or at least 30 minutes); this
allows the hydrolysis reaction to proceed before combining.
Preferably a composition containing surfactant and silica precursor
also includes an acid, preferably HCl. Preferably a composition
containing surfactant and silica precursor also includes an
alcohol, preferably the silica precursor is a silicon alkoxide and
the alcohol has the same hydrocarbon moiety as the alkoxide. The
higher the amount of surfactant and TEOS in the solution, the
thicker the resulting coating. The composition for forming a
mesoporous layer can be coated, preferably by dip coating or spray
coating, and then dried and heated in air, preferably at a
temperature of 10 to 500.degree. C. Dip coating typically results
in a coating thickness of about 1 .mu.m. The thickness of the
mesoporous silica layer is preferably at least 1 .mu.m, and in some
embodiments 1 to 5 .mu.m. Coatings less than 1 .mu.m result in
undesirably sparse nanotubes. Mesoporous coatings thicker than
about 5 .mu.m are undesirable because cracks will form during
drying which can lead to flaking (nonadherence). One dip coating is
preferred over multi-dips because a second coating will seal the
pore mouths of the first layer--thus the pores of the first layer
cannot be effectively used for nanotube growth. Templating agents
such as C.sub.16 EO.sub.18 can increase the size and spaciong of
nanotubes as compared with smaller agents.
Carbon nanotubes may be formed by pyrolysis of a carbon-containing
gas such as ethylene, acetylene or CO. Preferably the nanotubes are
grown at 600-1000.degree. C., with tube length increasing with
time. For higher purity, growth is conducted in alternating cycles
of tube growth and oxidation to remove amorphous carbon. If
desired, the nanotubes may be treated such as by heating in air to
form an oxidized surface. Preferably the surface is oxidized to a
sufficient extent to make the surface hydrophilic, preferably with
a static contact angle of less than 30.degree..
A catalytically active component or components (typically through
catalyst composition precursors which are subsequently treated to
produce a catalyst composition) can be applied directly on the
nanotubes or over intermediate layer(s) disposed over the
nanotubes. The as-grown carbon nanotubes are hydrophobic in nature,
thus, aqueous or other polar solvents containing metal or oxide
catalyst precursors absorb minimally on these nanotube sponges. For
this reason, surface treatment prior to dip coating is highly
desirable to modify the surface properties. In order to enhance the
absorption of catalyst precursors or other compositions, the wall
of the nanotube sponges may be oxidized, for example, at moderate
temperature in the presence of O.sub.2, etched in an acid solution
(preferably nitric acid), or exposed to a peroxide. After
oxidation, the uptake of the precursor solvents increases
dramatically. After dip coating, the substrate is annealed at high
temperature to remove the H.sub.2 O absorbed by capillary forces
within the sponge structure and to decompose the metal precursors.
Alternatively to dip coating, catalyst component or catalyst
component precursors can be applied by wash coating, vapor
depositing, electrolytically depositing or depositing in nonpolar
solvents.
The nanotubes can also be functionalized by treatment with a diene
or known functionalizing reagents.
The nanotube layer and/or a catalyst structure can be treated to
obtain a hydrophilic or hydrophobic surface depending on the
intended use.
Another approach for depositing a layer of thin oxide film on the
nanotube structure is to immerse the substrate into a surfactant
templated sol solution containing the ceramic and metal precursors.
In this case, the templated liquid crystals preferentially anchor
on the surface of the nanotubes. Subsequent drying and annealing at
high temperature removes the surfactant molecules, resulting in an
oxide layer with well-defined pore structures that adhere strongly
onto the nanotube surface. The physical properties of the oxide
formed depend on the surfactant/alcohol/water/precursor ratio.
Chemical reactions using the carbon-nanotube-containing catalysts
are also part of the invention. Examples of these reactions
include: synthesis of hydrocarbons from CO and H.sub.2, steam
reforming, acetylation, addition reactions, alkylation,
dealkylation, hydrodealkylation, reductive alkylation, amination,
aromatization, arylation, carbonylation, decarbonylation, reductive
carbonylation, carboxylation, reductive carboxylation, reductive
coupling, condensation, cracking, hydrocracking, cyclization,
cyclooligomerization, dehalogenation, dimerization, epoxidation,
esterification, exchange, halogenation, hydrohalogenation,
homologation, hydration, dehydration, hydrogenation,
dehydrogenation, hydrocarboxylation, hydroformylation,
hydrogenolysis, hydrometallation, hydrosilation, hydrolysis,
hydrotreating, hydrodesulferization/hydrodenitrogenation (HDS/HDN),
isomerization, methanol synthesis, methylation, demethylation,
metathesis, nitration, partial oxidation, polymerization,
reduction, steam and carbon dioxide reforming, sulfonation,
telomerization, transesterification, trimerization, water gas shift
(WGS), and reverse water gas shift (RWGS). Reactions can be in
liquid, gas, or liquid/gas, and gas/liquid/solid phase. Particular
advantages with the inventive catalyst structure are expected in
the reactions where heat and mass transfer limitations are
significant, such as liquid phase oxidation and hydrogenation
reactions.
The inventive carbon nanotube-containing structure can also be
advantageously used in aqueous phase and hydrothermal conditions,
for example, as a catalyst support. Examples of aqueous phase
reactions and hydrothermal reactions and conditions are described
in Elliott et al., U.S. Pat. Nos. 5,814,112, 5,977,013, 6,152,975
and 6,235,797, which are incorporated herein as if reproduced in
full below. Thus, the invention includes any of the aqueous phase
reactions described in the foregoing patents in combination with
the inventive catalysts.
EXAMPLES
Layered, aligned carbon nanotubes on metal and ceramic foams were
prepared and characterized. The figures show various SEM views of
various coated and uncoated foams. FIG. 1a is an SEM view of a
FeCrAlY alloy foam. FIG. 1b shows the same type of foam after
depositing carbon nanotubes. From the photomicrograph, one can see
chunks of aligned nanotubes coating the foam while leaving large
pores through the structure. Higher magnification views are shown
in FIGS. 1c and 1d. In FIG. 1d the carbon nanotubes appear curved
and wavy (kinked). Thus, the nanotubes have a local alignment (see,
e.g., FIGS. 1b, 1c) but a jumbled, high surface area orientation at
a very high magnification. Thus showing a preferred arrangement in
which nanotube alignment is observed at 2000.times. magnification
while substantial kinkiness is observed at 20,000.times.. The
interior of a nanotube-coated metal foam support was viewed by SEM
of a cross-sectional cut (not shown) demonstrated that the
technique was effective to cover surfaces throughout a large pore
support.
The effects of extending the period of nanotube growth conditions
can be seen by comparing FIGS. 2a with 2b (longer growth time) and
2c (longest growth time). Extensive nanotube growth may have the
effect of closing up the pore structure of a large pore support.
SEM views of carbon nanotubes on ceramic monolith at various
magnifications are shown in FIGS. 3a-3c. The appearance is very
similar to the growth on metal foams under similar conditions.
FIGS. 4a-4c show SEM views of an FeCrAlY foam that has been coated
(as described below) with an alumina layer, a mesoporous silica
layer, a layer of carbon nanotubes, and a surface layer of alumina.
The alignment of carbon nanotubes can be seen if we view nanotubes
perpendicular to the nanotube films. However, if nanotubes are
viewed from the top of the nanotube films, they appear random as
shown in FIG. 4b.
Preparation of Carbon Nanotube Coated Metal Foam
FeCrAlY intermetallic alloy foam (80 ppi, 85% porosity, purchased
from Porvair, Hendersonville, N.C.) was heat treated by exposing to
air at a temperature of at least 800.degree. C. (ramping rate of
20.degree. C./min, 2 h treatment). The heat treatment results in
the formation of a layer of alpha alumina on the surface due to
either outward diffusion of Al or inward diffusion of oxygen.
Fe/mesoporous silica was coated onto the foam, with a thickness
preferably ranging from 0.1 .mu.m to 5 .mu.m, using a dip-coating
method. This Fe/mesoporous silica was prepared from a templating
surfactant composition containing a mixture of alcohol, surfactant,
water, silica, and iron precursors. To enhance the adhesion between
the metal foam and the Fe/mesoporous silica catalyst, a dense and
pinhole-free interfacial layer was first coated onto the oxidized
FeCrAlY foam by metal organic chemical vapor deposition (MOCVD).
This interfacial layer can be Al.sub.2 O.sub.3, Al.sub.2 O.sub.3
+SiO.sub.2, or TiO.sub.2, etc. For example, when TiO.sub.2 was
coated, titanium isopropoxide (Strem Chemical, Newburyport, Mass.)
was vapor deposited at a temperature ranging from 250 to
900.degree. C. at a pressure of 0.1 to 100 torr. Titania coatings
with excellent adhesion to the foam were obtained at a deposition
temperature of 600.degree. C. and a reactor pressure of 3 torr.
This layer not only increases the adhesion between metal foam and
the Fe/mesoporous silica, it also protects the FeCrAlY from
corrosion during the subsequent carbon nanotube growth and
reaction.
A layer of Fe/mesoporous silica was dip-coated onto the foam from a
templating surfactant composition to yield a high surface area
silica layer containing dispersed Fe particles. High surface area
ensures a high carbon nanotube growth rate and a high carbon
nanotube density per surface area. Upon high temperature
calcination, the precursors decomposed to form a layer of
Fe/SiO.sub.2, typically with a thickness of 0.5-3 .mu.m, which
served as the seeding layer for carbon nanotube growth. It should
be noted that preparation of this seeding layer is not limited to
the one step dip-coating method with a Fe containing templating
surfactant composition, as mentioned above. It can also be done by
conventional methods such as incipient wetness impregnation or
vapor phase impregnation of Fe precursors on a pre-deposited silica
layer. In addition, the choice of catalytic active metals is not
limited to Fe, other metals such as Co and Ni have been
demonstrated to be able to decompose gas phase carbon containing
molecules to form carbon nanotubes.
As compared to using a wash coat slurry formed from preformed
Fe/SiO.sub.2 powder onto the metal foam, dip coating the foam
substrate with a sol gel solution has unexpectedly been found to
have significant advantages. The dip coating approach uses direct
gelation method, where the sol solution is gelled and dried onto
the metal foam. During the drying and gelation process, the
mesoporous materials adhere strongly onto the metal foam, thus
providing an intimate contact between the foam and the Fe/SiO.sub.2
coating without any void micropores on the interface which would
hamper heat transfer in the final engineered catalyst during
reaction.
The carbon nanotube growth was carried out at temperatures of
600-1000.degree. C., depending on the carbon sources and the
catalysts. A carbon containing gas source such as ethylene,
acetylene, CO, was introduced to the substrate for 5-20 min, where
the well-aligned carbon nanotubes were formed by reaction between
the C gas source and the Fe particles. In this particular example,
ethylene was used. The growth rate and length of carbon nanotubes
were controlled by both temperature and duration. During the
growth, the gas was introduced into the chamber at a period of not
more than 20 min, since longer growth time resulted in deposition
of amorphous carbon and randomly aligned tubes. If necessary, a
growth cycle was conducted. Between the growths, O.sub.2 /N.sub.2
with<500 ppm of O.sub.2 was introduced to the chamber for 5 min
to oxidize any amorphous carbon deposited. A growth rate of 0.5-5
.mu.m/min was typically observed.
The as grown carbon nanotubes are hydrophobic in nature, thus,
conventional aqueous solutions containing metal or metal oxide
catalyst precursors adsorb minimally on these nanotubes sponges.
For this reason, surface treatment prior to dip coating of
catalytic components is highly desirable to modify surface
properties. In order to enhance the adsorption of catalyst
precursors, the nanotube sponges can be either oxidized at moderate
temperature (such as 450.degree. C.) in the presence of O.sub.2 for
5 min or etching in a nitric acid solution (at room temperature).
After treated with these procedures, the uptake of the precursor
solvents increases dramatically. After dip coating, the substrate
is annealed at high temperature to remove the H.sub.2 O absorbed by
capillary forces within the sponge structure and to decompose the
metal precursors.
In order to further modify the surface of carbon nanotubes, a layer
of thin oxide film can be deposited on the treated carbon nanotubes
by immersing the substrate into a surfactant templated sol solution
containing the ceramic and metal precursors. In this case,
templated liquid crystals preferentially anchor on the surface of
the nanotubes. Subsequent drying and annealing at high temperature
removes the surfactant molecules, resulting in an oxide layer with
well-defined pore structures that adhere strongly onto the nanotube
surface. The physical properties of the oxide formed depend on the
surfactant/alcohol/water/precursor ratio.
In an alternate method, nano-size Al.sub.2 O.sub.3 particles were
deposited by dipping carbon nanotube coated metal foams in an
Al.sub.2 O.sub.3 colloid solution. The weight percentage of the
colloid solution varied from 5% to 20% to control the Al.sub.2
O.sub.3 loading in the carbon nanotube sponge. The solvent of the
colloid solution also varied from pure water to 25 wt. % water/75
wt. % ethanol to control the surface tension of the colloid
solution. After dipping in the colloid solution for >1 min, the
coated metal foams were removed from the solution, and the excess
solution was then removed on filter papers. The coated metal foams
were rapidly dried in a few minutes under vacuum (<1 torr) at
room temperature. Low temperature annealing (e.g. 450.degree. C. in
air for 0.5 hr) was necessary to completely remove the solvent.
FIGS. 3a and 3b show typical SEM images of an
alumina-coated-carbon-nanotube-coated-metal-foam that was coated
using 8 wt. % Al.sub.2 O.sub.3 colloid solution.
Preparation of Fischer-Tropsch Catalysts
Using the above method, an engineered catalyst was fabricated. The
FeCrAlY intermetallic foam (80 ppi, 85% porosity) with the
dimensions of 0.30".times.1.4".times.0.06" was first oxidized at
900.degree. C. in air for 2 h, and then coated with a submicron
layer of Al.sub.2 O.sub.3 using MOCVD. The MOCVD was carried out
using aluminum isopropoxide as the precursor with N.sub.2 carrier
gas in an oxidizing environment containing 14 vol % of O.sub.2
under 5 Torr at 850.degree. C. The aluminum isopropoxide precursor
was stored in a bubbler where the vapor pressure was controlled by
changing the bubbler temperature. In this case, the temperature was
controlled at 106.degree. C. The coated foam was cooled to room
temperature after the MOCVD.
A surfactant templated solution was prepared. according to weight
ratio of: C.sub.16 EO.sub.10 (polyoxyethylene 10 cetyl
ether):ETOH:TEOS:HCl:Fe(NO.sub.3).sub.2.9H.sub.2 O:H.sub.2 O of
17.5:75:40:1:40:100. The preparation began with two separate
solutions, i.e. an alcohol based and an aqueous solution, prepared
according to the weight ratio mentioned above, then mixed together
during the final stirring step.
To prepare the alcohol-based solution, the C.sub.16 EO.sub.10
surfactant was first dissolved in ethanol under continuous stirring
for 1 h at 40.degree. C. on a hot/stir plate. The heat setting on
the hot plate was turned off after 1 h stirring before TEOS
(tetraethylorthosilicate) and 12 M HCl were consecutively added
into the solution. Between the additions, the solution was aged and
stirred for a 1 h period. The C.sub.16 EO.sub.10 (polyoxyethylene
10 cetyl ether): ETOH:TEOS:HCl ratio was 17.5:75:40:1. (Another
preparation used 3.16 mL H2O, 8.08 g EtOH, 0.173 g HCl, 4.075 g
C.sub.16 EO.sub.10, and 8 mL TEOS. Sometimes, the H.sub.2 O was
replaced by EtOH, and no HCl was used.
Separately, an aqueous Fe nitrate solution was prepared according
to a ratio of: Fe(NO.sub.3).sub.2.9H.sub.2 O:H.sub.2 O of 40:100.
The Fe precursor was first dissolved in de-ionized water and
stirred for at least 1 h. Both solutions were mixed together and
stirred for another hour prior to dip coating onto the foam. The
excess solution was removed by absorbing onto a filter paper, then
the substrate was calcined at a ratio of 1.degree. C./min from room
temperature to 450.degree. C., and held isothermally at that
temperature for 1 h under air before cooling to room temperature.
At this stage, the substrate was ready for the carbon nanotube
growth. The substrate was loaded into a 1.25" OD quartz reactor,
heated under 500 sccm of N.sub.2 flow from room temperature to
700.degree. C. 500 sccm of ethylene was introduced into the flow
reactor for three-20 min periods, with 5 min O.sub.2 /N.sub.2
(.about.200 ppm O.sub.2) purge between those periods. After the
combined growth time of 60 min, the substrate was cooled down from
700.degree. C. to 450.degree. C. under N.sub.2. At 450.degree. C.,
air was introduced to the growth chamber for 5 min to oxidize the
surface of the carbon nanotube before further cooling to room
temperature under N.sub.2. The substrate was then dip coated with a
colloid alumina solution containing approximately 5 wt % of
Al.sub.2 O.sub.3 which was prepared by mixing a PQ alumina
colloidal sol (PQ Corp, AL20DW Lot #30-001598, Ashland, Mass.) with
a 1:3 H.sub.2 O/EtOH solvent, followed by drying at 110.degree. C.
before calcining at 350.degree. C. for 3 h in air. The substrate
was then dip coated with an aqueous solution containing cobalt and
rhenium precursors. Cobalt nitrate hexahydrate and pherrennic acid
were used as the precursors, they were co-dissolved in the 4.883 M
cobalt and rhenium (Co+Re) solution with Co/Re molar ratio of
29.79, dried at 110.degree. C. then calcined at 350 C. for 3 h. A
catalyst (run ID of MD153 in Table 1 on CNT/FeCrAlY substrate) was
prepared containing 0.0431 g Co-Re/Alumina with 37 wt % Co4 wt %
Re. The weights of FeCrAlY foam substrate and carbon nanotubes are
0.3799 g and 0.0675 g, respectively.
Preparation of Fischer-Tropsch Catalysts without Carbon
Nanotubes
Co/Re/alumina catalysts were also prepared on the same FeCrAlY
substrate without a carbon nanotube layer. The FeCrAlY
intermetallic foam (80 ppi, 85% porosity) with the dimensions of
0.30".times.1.4".times.0.06" was first oxidized at 900.degree. C.
in air for 2 h, then was coated with a submicron layer of Al.sub.2
O.sub.3 using MOCVD at 850.degree. C. The MOCVD was carried out
using aluminum isopropoxide as the precursor with N.sub.2 carrier
gas in an oxidizing environment containing 14 vol % of O.sub.2
under 5 Torr at 850.degree. C. The aluminum isopropoxide precursor
was stored in a bubbler where the vapor pressure was controlled by
changing the bubbler temperature. In this case, the temperature was
controlled at 106.degree. C. The coated foam was cooled to room
temperature after the MOCVD. The substrate was then dip coated with
a colloid alumina solution containing approximately 5 wt % of
Al.sub.2 O.sub.3 which was prepared by mixing a PQ alumina
colloidal sol (PQ Corp, Lot #30-001598, Ashland, Mass.) with a 1:3
H.sub.2 O/EtOH solvent, followed by drying at 110.degree. C. before
calcining at 350.degree. C. for 3 h in air. Upon cooling to room
temperature, the substrate was dip coated with an aqueous solution
containing cobalt and rhenium precursors. Cobalt nitrate
hexahydrate and pherrennic acid precursors were co-dissolved in the
4.883 M cobalt and rhenium solution with Co/Re ratio of 29.79,
dried at 110.degree. C. then calcined at 350 C for 3 h. A catalyst
(run ID of MD157 in Table 1 on FeCrAlY substrate) was prepared
containing 0.0662 g Co/Re/Alumina with 50 wt % Co5 wt % Re.
Preparation of Fischer-Tropsch Catalysts without Carbon Nanotubes
on Other Metal Substrates
Fischer-Tropsch catalysts were also prepared on various other metal
foam substrates (Cu, stainless steel, GPM) with the dimensions of
0.30".times.1.4".times.0.06" without carbon nanotube layer. These
metal substrates were purchased from Porvair (Hendersonville, N.C.)
with 80 ppi pore density and 85% porosity except that GPM (FeCrAlY)
has a much higher pore density (>400 ppi with an average pore
size of 30 microns) and lower porosity (70%). These catalysts were
prepared as follows. First, acidic gamma-alumina support powder
(Engelhard) was ground and sieved to between 70- and 100-mesh (150
to 220-micron), and calcined (stabilized) at 500.degree. C. for
several hours. This powder was then impregnated with a solution
containing cobalt nitrate hexahydrate and pherenic acid precursors,
present in desired concentrations to produce a 20-wt % cobalt, 4-wt
% Re on alumina catalyst. The precursor solution was prepared in
such a manner as to saturate the pore volume of the alumina support
without over saturation of the alumina support. This powder was
then dried in a vacuum oven at 100.degree. C. for at least 4 hours,
followed by drying at 100.degree. C. for at least 12-hours. The
powder was then calcined by heating at 350.degree. C. for at least
3 hours. A portion of the powder was then combined with distilled
water in a water-to-catalyst weight ratio of at least 2.5 to
produce a catalyst slurry. This catalyst slurry is then placed in a
container with inert grinding media balls and placed on a rotating
device for at least 24 hours. This slurry was then ready to coat a
pre-treated metal foam type support. The metal foam pretreatment
consisted of cleaning successively in dichloromethane and acetone
solvents in a water bath submersed in a sonication device to
agitate the solvent within the foam. Optionally, the metal surface
of the monolith may then be roughened by etching with acid. If this
is desired, the metal foam is submerged in 0.1-molar nitric acid,
and placed in a sonication device. The metal foam was then rinsed
in distilled water and dried at about 100.degree. C. The metal
foams, except the Cu foam, were then coated with a layer of alumina
using a metal organic chemical vapor deposition (MOCVD) technique.
Cu foam was used after the cleaning without the CVD Al.sub.2
O.sub.3 coating. The CVD system has a horizontal, hot-wall reactor
with three precursor sources. The CVD coatings are performed at a
deposition temperature between 600.degree. C.-850.degree. C. and
reactor pressure of 5 torr. Aluminum iso-propoxide was used as the
aluminum precursor. This precursor is stored in a quartz container
maintained at 106.degree. C. during deposition, which produces a
vapor that is carried into the CVD reactor by a flow of nitrogen
carrier gas for about 60 minutes. Air was then used to oxidize the
aluminum precursor to alumina. Typical thickness of the alumina
coatings is about 0.5 .mu.m. This pretreated metal support foam was
then coated with the catalyst slurry by dip coating. The metal foam
was then dried in flowing air or nitrogen at room temperature while
continuously rotating the metal foam in such a way as to create a
uniform coverage of the dried catalyst slurry layer. The metal foam
was then dried at 90.degree. C. for at least 1-hour, heated slowly
to 120.degree. C. over the course of at least-hour, dried further
at 120.degree. C. for at least 2 hours, and then heated to
350.degree. C. and calcined for at least 3 hours. The weights of
alumina supported Co-Re powder catalyst on the metal foam are
listed in Table 1.
Catalyst Activity Comparision
The engineered catalysts aforementioned were placed inside the
reaction chamber and activated (or reduced) prior to reaction by
heating to about 350.degree. C. to 400.degree. C. and under flow of
a hydrogen-containing stream of about 10 to 20% (by mole or volume)
hydrogen in an inert carrier gas (such as nitrogen or helium) at a
flow rate of at least 20 cc/min (measured at 273 K and 1 atm) for
at least 2-hours. The catalyst was then allowed to cool to reaction
temperatures, about 266.degree. C. The catalyst was then exposed to
a feed gas comprised of H.sub.2 and CO in a desired ratio of moles
of H.sub.2 per mole of CO (2/1). The feed gas flow rate is
controllable to allow for precise generation of a desired contact
time (250 msec). The reaction products were then analyzed to
evaluate the conversion of CO and the selectivity towards certain
products, such as methane. The reaction was conducted at a pressure
of 16 atmospheres. The results are shown in Table 1. Although
carbon nanotube containing catalyst has much lower loading of
active components, it has much higher specific activity than other
catalysts without carbon nanotubes while maintaining similar
methane selectivity.
TABLE 1 Comparison of Fischer-Tropsch Activity with and without
Carbon Nanotubes*** Temp Weight of Co + CO CH4 specific ID
(.degree. C.) Support Catalyst** Re + Al.sub.2 O.sub.3 Conv.
selectivity activity* MD 266 Cu 20% Co-4% 0.174 46% 30% 1070 144 Re
MD 260 SS 20% Co-4% 0.182 50% 32% 1110 145 Re MD 266 GPM 20% Co-4%
0.174 60% 26% 1380 151 Re MD 266 CNT/ 37% Co-4% 0.0493 g 42% 27%
2362 153 FeCrAlY Re MD 265 FeCrAlY 50% Co-5% 0.662 g 20% 31% 645
157 Re *mmol CO converted per gram of cobalt (total) per hour **The
balance is Al.sub.2 O.sub.3 ***All experiments were conducted at
16-atm, H.sub.2 /CO = 2, 250-msec contact time. Listed performance
values were gathered after 96- to 120-hrs TOS when catalyst reached
steady state performance.
* * * * *